Method of controlling fluorescent lamp output

ABSTRACT

A method and apparatus for controlling the output of the fluorescent lamps in an environmental growth chamber to produce uniform or predetermined light levels within chambers of the type including means for conducting cooling air across the lamps. A temperature for the air is selected which is different than the temperature at which maximum light output occurs from the lamps and which corresponds to a desired light output. At at least periodic intervals thereafter, the light output from the lamps is measured and if by light sensitive means the output is above or below that desired, the cooling air temperature is manually or automatically adjusted to bring the output to the desired level.

United States Patent Noel Davis c/o Integrated Dev. and Manufacturing Co., 510 East Washington St., Chagin Falls,

[72] lnventor Ohio 44022 [21] Appl. No. 26,357 [22] Filed Apr. 7, I970 [45] Patented [73] Assignee [54] METHOD OF CONTROLLING FLUORESCENT [56] References Cited UNITED STATES PATENTS 3,296,488 1/1967 Taylor 313/13 X 3,309,565 3/1967 Clark et al. 315/117 3,359,454 12/1967 Scheppe 315/117 3,393,728 7/1968 Davis 165/48 Primary ExaminerCarroll B. Dority, Jr. Attorney-Meyer, Tilberry and Body ABSTRACT: A method and apparatus for controlling the output of the fluorescent lamps in an environmental growth chamber to produce uniformsor predetermined light levels within chambers of the type including means for conducting cooling air across the lamps. A temperature for the air is selected which is different than the temperature at which maximum light output occurs from the lamps and which corresponds to a desired light output. At at least periodic intervals thereafter, the light output from the lamps is measured and if by light sensitive means the output is above or below that desired, the cooling air temperature is manually or automatically adjusted to bring the output to the desired level.

PATENTED SEP] 4 I971 3,504 500 -20 0 2O 4O 6O 80 I00 I20 I40 MINIMUM TEMPERATURE OF BULB WALL DEG.F.

INVENTOR.

/ Z" I N062 0/1 W5 BY W AW METHOD OF CONTROLLING FLUORESCENT LAMP OUTPUT The invention is particularly adapted for use in controlled environment chambers of the type used for performing biological experiments and will be described with particular reference thereto; however, it will be appreciated the invention is capable of broader application and could be used in many other devices or installations.

During the performance of biological experiments it is particularly important that the environmental conditions be closely maintained throughout the experiments. Chambers capable of maintaining temperature, humidity, and airflow at closely controlled levels have been developed and are in widespread use. It has generally not been possible however, to maintain the light intensity within the chambers at extreme close levels.

In the past, most chamber designs have been directed toward obtaining a maximum level of light intensity. This has been accomplished by use of large numbers of closely spaced fluorescent lamps together with highly reflective lamp chambers. Additionally, to prevent the heat from the lamps from unduly affecting the temperature levels within the test chambers, the lamp chambers were sealed from the test chamber by Plexiglas or the like and cooling air circulated over the lamps. Light output controls, if used, generally consisted of relatively expensive and bulky lamp supply voltage control systems which were adjusted to maintain the lamps at a maximum light output level.

The voltage control systems, in addition to their bulk and expense, often resulted in a shorter than normal life for the lamps. Further, with the voltage control system, the light intensity could not be easily maintained at any one level. It was found that the difficulty was due to the natural decay of the lamps throughout their life, together with ambient cooling air temperature variations.

The present invention provides a method and apparatus for controlling the light level extremely accurately for extended periods of time. The invention is based on the known phenomenon that, with fluorescent lamps, the light output varies with the temperature of the coldest spot on the lamp wall.

in accordance with the invention, there is provided a method of controlling the light output of fluorescent lamps which includes the steps of (a) selecting a first temperature which is different than the temperature at which maximum light output occurs from the lamps and which corresponds to a desired light output from the lamps; (b) directing cooling air over the lamps at the selected temperature; (c) at least periodic intervals during operation of the lamps, measuring their light output level; and (d) if the light output is below the desired output, varying the temperature of the cooling air in a direction toward the temperature at which maximum light output occurs until the light output again reaches the desired level.

Since the shape of the curve of light output versus the temperature of the coldest spot on the lamp wall is generally bell shaped, the originally selected temperature can be either above or below the temperature of maximum output. The selection as to whether the original temperature is above or below, is not particularly significant; however when the temperature selected is below the maximum the load on the temperature control apparatus of the associated chamber is less and the life expectancy of the lamps is somewhat greater.

Further in accordance with the invention, there is provided for use with a fluorescent light source and means for maintain ing at least a spot in such sources at a predetermined temperature light sensitive means operative to vary the temperature of such spot so as to maintain the light output constant as the sources age.

A primary object of the invention is the provision of an improved method of operating fluorescent lamps to control the light output level.

Another object is the provision of method of apparatus whereby the light output level of fluorescent lamps can be closely controlled throughout their life.

Yet another object is the provision of an arrangement for operating fluorescent lamps in a manner which permits ready variation in light output level without varying the voltage or current input.

Still another object is the provision of method and apparatus for controlling the closed circuit lamp cooling system of environmental growth chambers and the like to provide a desired output light level.

. A further object is the provision of method and apparatus for maintaining the light level constant within a closed chamber by regulation of the lamp cooling system in response to changes due to aging or otherwise.

The above and other objects and advantages will become apparent from the following description when read in conjunction with the accompanying drawings wherein:

FIG. 1 is a pictorial view of the upper portion of an environmental growth control chamber, partial in section, showing the lamp chamber and its associated cooling chamber; and,

FIG. 2 is a graph of a family of lines showing light output versus the minimum temperature of the bulb wall for a typical fluorescent lamp.

Referring more particularly to the drawings wherein the showings are for the purpose of explaining the preferred embodiment of the invention only and not for the purpose of limiting same.

FIG. 1 shows, partially in section, the upper portion of a typical environmental growth chamber installation comprised of a test chamber portion 10, an enclosed lamp chamber 12 and a superposed lamp cooling chamber l4. The precise details of the growth chamber construction form no part of the present invention and are illustrated merely for the purpose of ex plaining one particular environment in which the invention is particularly adapted for use.

The test chamber 10 comprises vertically extending walls 16 provided with suitable doors and access openings not shown. In the embodiment under consideration, the walls 16 are formed with an insulation core 18 such as expanded styrene foam covered with polished sheet aluminum 20.

Positioned within the test chamber 10 and adapted to maintain closely controlled conditions of temperature and humidity are vertical sidewall temperature and humidity control units 22 each of which includes means for maintaining a constant and uniform circulation of conditioned air to the test chamber 10. Additionally, the chambers usually include heating and cooling coils together with humidifying and dehumidifying devices.

The lamp chamber 12 is positioned over the open upper end of the test chamber 10 and includes a sheet metal housing 24 having a piece of Plexiglas or similar transparent sheet material 26 positioned across its lower end to seal the lamp chamber from the test chamber. Positioned within the lamp chamber and extending horizontally therethrough are a substantial number of closely spaced fluorescent lamps 28. The lamps, in the embodiment under consideration, extend horizontally above the test chamber and are spaced closely together throughout the width ofthe lamp chamber.

As can be appreciated, the large number of lamps in the lamp chamber produces a substantial heat buildup. in order to prevent the heat buildup from unduly effecting the temperatures or increasing the load on the air-conditioning equipment within the test chamber, a lamp-cooling arrangement is provided. The cooling means includes the closed cooling chamber M which has heat exchange coils 30, 32 positioned therein. Fans 34 and 36 are positioned at opposite ends of the lamp chamber 12 and arranged to continuously withdraw air from opposite ends of the lamp chamber and direct upwardly as shown by the arrows to the cooling chamber and the heat exchange coils 30, 32. Thereafter the cooled air is directed downwardly to the center opening 38 against the lamps 28. This maintains the lamp chamber relatively cool and conducts away the excess heat generated therein.

In the embodiment under consideration, heat exchangers 30, 32 are standard finned coil units and a chilled water coolant is circulated therethrough by inlet lines 40, 42 respectively. The coolant is thereafter returned to the coolant source through lines 44 and 46 respectively.

In order to maintain the air passing through the lamp chamber at a predetermined temperature level, the supply lines to the heat exchange coils 30, 32 are provided with control valves 48, 50 which are controlled by temperature-sensing elements 52, 54 positioned within the airstream discharging from the lamp chamber. These elements and valves function to control the flow of coolant to the heat exchange coils to maintain a desired temperature within the lamp chamber. This type of close circuit lamp chamber cooling for environmental growth chamber is known and, is shown for example, in my prior U.S. Pat. No. 3,393,728. As disclosed, the lamps are preferably provided with small clip on fin members, such as shown at 56, which are positioned in the air discharging from the cooling chamber against the lamps. According to this patent these fins conduct heat away from a spot on the lamps and maintain a particular spot on the lamps at substantially the cooling air temperature to obtain maximum light output from the lamps. The elements 52,54 control valves 48, 50 so that the temperature of the air supplied to the lamp chamber is maintained at a level required to obtain maximum light output from the lamps. In accordance with the invention, the elements 52, 54 or the valves 48, 50 or both are either manually or automatically controlled so that this temperature can be readily adjusted to a valve below the maximum light output of new lamps.

Referring in particular to FIG. 2, there is shown a graph of light output versus minimum bulb wall temperature for a typical fluorescent lamp. The curve will of course vary depending upon the particular details or structure of the lamp however, the graph shown is typical for substantially all fluorescent tubes. Line 60 shows the relationship between temperature and light output for a new lamp. Note that the light output goes from substantially at a temperature 0 F. to a maximum at approximately 100 F. Thereafter, the light output decreases with an increase in bulb wall temperature. This decrease is substantially linear. Lines 62, 64 and 66 are typical of the family of lines which show the output versus bulb wall temperature as the lamp ages. Note that the general shape of the lines remains the same. However, the light output at any particular temperature tends to decrease as the bulb ages.

if it is desired to maintain maximum light output from the unit then the bulb is continuously maintained at a temperature around 100 F; however, as can be appreciated as the bulb ages the maximum light output continuously decreases. This can be of a distinct disadvantage considering that experiments may run for several months or, it may be desirable to duplicate the exact lighting conditions of a previous experiment. In such case, this is not possible with the prior apparatus and methods of operating. According to the present invention, however it is possible by slightly sacrificing the maximum light output of the lamps to obtain a predetermined or a constant level of light output throughout the life of the lamps so that it is simple to reduplicate prior light levels or to maintain a continuous light level.

Referring particularly to FIG. 2 it will be seen that lines 70, 72 represent a constant light output level. For example, when the bulb is new, if an original temperature slightly above 80 F. is selected as the control point, the light level will be approximately 88 percent of maximum. At periodic points thereafter the light level within the chamber is measured and the temperature to which the control valves 48 and 50 are adjusted is increased if the light level has decreased. That is, the temperature is shifted to move the light output of the lamp in a direction toward its maximum. Thus returning it to the same level of light output as originally. By continuous monitoring and readjustment of the temperature throughout the life of the bulb or throughout the experiment, it is possible to maintain the light level substantially constant.

The same results can be achieved by selecting a light or temperature level which is above the level at which maximum light output occurs. For example, if when the bulb is new, the temperature point indicated by reference numeral 74 is selected and the light output of the lamps continuously monitored thereafter, a drop in light output can be overcome by slowly decreasing the temperature of the cooling air passing over the bulbs until a new lower temperature is reached such as at point 76 wherein the light output is again at the desired level.

This method of operation provides an extremely simple and easily carried out method whereby light output can be maintained at desired levels.

The same method can be used for obtaining a dimming effect on the lamps. For example, if it were desired to operate the chamber or to run a test at an extremely low light level the lamp-cooling system can be used to provide a uniform dimming effect on all lamps without the need for switching out particular lamps or providing a voltage or current control system. Note that merely by selecting a lower lamp-cooling chamber temperature any desired light output can be achieved. Clearly the same method can be used for obtaining a programmed light level throughout a test. Merely by varying the cooling temperature according to a predetermined pattern the light level within the chamber can be made to vary accordingly.

Many types of apparatus could be utilized in the practice of the invention and, for example, could continuously monitor the light output and gradually and continuously change the temperature at the light output varied due to lamp aging alternatively, a periodic monitoring system can be utilized and the light shifted at periodic intervals.

For example, a light-sensitive element 80 is provided which operates through conventional control apparatus 81 to vary the valves 48, 50 so as to vary the flow of coolant as the light output tends to vary thus preventing such variation. Alternatively, the element 80 can act to vary the temperature setting of elements 52, 54 from their preset adjustment for a light output below the maximum.

The invention has been described in relation to preferred embodiments. Obviously modifications and alterations will occur to others upon a reading and understand of the specification and it is my intention to include all such modifications and alterations insofar as they come within the scope of the claims.

lclaim:

l. A method of controlling the light output of fluorescent lamps on apparatus of the type including a lamp chamber and means for supplying cooling air to the lamp chamber including the steps of:

a. selecting a first temperature which is different than the temperature at which maximum light output occurs from the lamps and which corresponds to a desired light output from the lamps;

b. directing cooling air over the lamps at the selected temperature;

c. at least periodic intervals during operation of the lamps,

measuring their light output level; and,

d. if the light output is below the desired output, varying the temperature of the cooling air in a direction toward the temperature at which maximum light output occurs until the light output again reaches the desired level.

2. The method as set forth in claim l wherein the first temperature selected is below the temperature at which maximum light output occurs.

3. The method as set forth in claim I wherein the cooling air is continuously directed over said lamps, and wherein the first temperature selected is below the temperature at which maximum light output occurs.

4. In a light source of the type including a fluorescent lamp chamber and means for supplying cooling air to the lamp chamber and wherein the fluorescent lamps have a light output which varies with the temperature of at least a spot on the surface of the lamp, the improvement which comprises: means for maintaining the temperature of the spot at a normal temperature below the maximum light output of the lamps, means for measuring the light output of the lamps and means for varying the temperature of the spot on the lamps as the light output tends to change and maintain the light output at a constant level.

5. The improvement of claim 4 wherein said means include a light-sensitive element positioned adjacent said lamps and means operatively associated with said element and said cooling air means for varying the temperature of said cooling air in response to changes in light output.

6 The improvement of claim 4 wherein said means include a temperature-sensitive element operatively associated with said cooling air, said element as a normal setting maintaining the temperature of said cooling air below the maximum light output of said lamps; and, means for varying said temperature setting so as to change the temperature of said cooling air as the light output of said lamps tend to vary.

7. The improvement of claim 5 wherein said normal temperature is below the maximum light output temperature and said light-sensitive element varies said temperature in inverse proportion to said light output.

8. The improvement of claim 5 wherein said normal temperature is above the maximum light output temperature and said light-sensitive element varies said temperature in direct proportion to said light output. 

1. A method of controlling the light output of fluorescent lamps on apparatus of the type including a lamp chamber and means for supplying cooling air to the lamp chamber including the steps of: a. selecting a first temperature which is different than the temperature at which maximum light output occurs from the lamps and which corresponds to a desired light output from the lamps; b. directing cooling air over the lamps at the selected temperature; c. at least periodic intervals during operation of the lamps, measuring their light output level; and, d. if the light output is below the desired output, varying the temperature of the cooling air in a direction toward the temperature at which maximum light output occurs until the light output again reaches the desired level.
 2. The method as set forth in claim 1 wherein the first temperature selected is below the temperature at which maximum light output occurs.
 3. The method as set forth in claim 1 wherein the cooling air is continuously directed over said lamps, and wherein the first temperature selected is below the temperature at which maximum light output occurs.
 4. In a light source of the type including a fluorescent lamp chamber and means for supplying cooling air to the lamp chamber and wherein the fluorescent lamps have a light output which varies with the temperature of at least a spot on the surface of the lamp, the improvement which comprises: means for maintaining the temperature of the spot at a normal temperature below the maximum light output of the lamps, means for measuring the light output of the lamps and means for varying the temperature of the spot on the lamps as the light output tends to change and maintain the light output at a constant level.
 5. The improvement of claim 4 wherein said means include a light-sensitive element positioned adjacent said lamps and means operatively associated with said element and said cooling air means for varying the temperature of said cooling air in response to changes in light output.
 6. The improvement of claim 4 wherein said means include a temperature-sensitive element operatively associated with said cooling air, said element as a normal setting maintaining the temperature of said cooling air below the maximum light output of said lamps; and, means for varying said temperature setting so as to change the temperature of said cooling air as the light output of said lamps tend to vary.
 7. The improvement of claim 5 wherein said normal temperature is below the maximum light output temperature and said light-sensitive element varies said temperature in inverse proportion to said light output.
 8. The improvement of claim 5 wherein said normal temperature is above the maximum light output temperature and said light-sensitive element varies said temperature in direct proportion to said light output. 